Re: [Qemu-devel] [RFC] Block device size rounding

[Peter Crosthwaite]

On Mon, Oct 12, 2015 at 9:26 AM, Eric Blake <address@hidden> wrote:
> On 10/12/2015 09:56 AM, John Snow wrote:
>
>>> What is the correct action here though? If the file is writeable should
>>> we just allow the device to extend its size? Is that possible already?
>>> Just zero-pad read-only?
>>>
>>
>> Read-only seems like an easy case of append zeroes.
>
> Yes, allowing read-only with append-zero behavior seems sane.
>
>>
>> Read-write ... well, we can't write-protect just half of a 512k block.
>
>> Probably just forcibly increasing the size on RW or refusing to use the
>> file altogether are probably the sane deterministic things we want.
>
> I'd lean towards outright rejection if the file size isn't up to snuff
> for use as read-write.  Forcibly increasing the size (done
> unconditionally) still feels like magic, and may not be possible if the
> size is due to something backed by a block device rather than a file.
>

Inability to extend is easily detectable and can become a failure mode
in it's own right. If we cant extend the file perhaps we can just
LOG_UNIMP the data writes? Having to include in your user instructions
"dd your already-on-SATA file system to this container just so it can
work for SD" is a pain.

Regards,
Peter

> --
> Eric Blake   eblake redhat com    +1-919-301-3266
> Libvirt virtualization library http://libvirt.org
>